What the pre-edge of the S3 state can reveal about the photosynthetic O2 formation mechanism and possible differences between procaryotes an

The S3 state of the Mn4Ca cluster of nature’s water-splitting engine, Photosystem II, is the last intermediate before O2 formation. Recently, different conformations of the S3 state (MnIV4 with S = 3 or S = 6)1–3 have been presented using EPR spectroscopy and proposed to be intermediates of the catalytic cycle. The heterogeneity is dependent on the treatment of Photosystem II and interestingly, is species-dependent i.e., different S3 configurations have been proposed for procaryotes (low spin S3 form) and eucaryotes (high spin S3 form). Based on DFT it was proposed that the two different configurations of S3 would lead to two different mechanisms of O2 formation in the next step (oxo-oxyl mechanism vs MnV nucleophilic attack). The two different S3 conformers present distinct pre-edge spectra. We propose to test experimentally this intriguing hypothesis by measuring the pre-edge of different conformations of the S3 state using two different organisms and different treatments.